drm/radeon/evergreen_cs: Fix misnaming issues surrounding 'p' param

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/evergreen_cs.c:1026: warning: Function parameter or member 'p' not described in 'evergreen_cs_packet_parse_vline'
 drivers/gpu/drm/radeon/evergreen_cs.c:1026: warning: Excess function parameter 'parser' description in 'evergreen_cs_packet_parse_vline'
 drivers/gpu/drm/radeon/evergreen_cs.c:1095: warning: Function parameter or member 'p' not described in 'evergreen_cs_handle_reg'
 drivers/gpu/drm/radeon/evergreen_cs.c:1095: warning: Excess function parameter 'parser' description in 'evergreen_cs_handle_reg'
 drivers/gpu/drm/radeon/evergreen_cs.c:1757: warning: Function parameter or member 'p' not described in 'evergreen_is_safe_reg'
 drivers/gpu/drm/radeon/evergreen_cs.c:1757: warning: Excess function parameter 'parser' description in 'evergreen_is_safe_reg'

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>